1. Field of Invention
The present invention relates to a stent delivery system. More particularly, the present invention relates to a stent delivery system that allows a stent to be released in a controlled manner upon its deployment.
2. Description of the Prior Art
Stents are well known and widely used in medical procedures. Stents are frequently used to treat various organs and vessels in the vascular system. In particular, stents are most commonly used to treat vascular diseases using endovascular procedures. Patients who suffer from vascular diseases typically develop stenosis of various arteries, i.e., blood vessels that are clogged or narrowed by substances that restrict blood flow. Traditionally, operations such as bypass surgery have been performed to treat the stenosis of arteries. Bypass surgery involves opening of the chest cavity, which is a very invasive procedure for patients.
The development of fluoroscopy was one of the first uses of non-invasive medical procedures. Fluoroscopy allows medical personnel to see internal organs and blood vessels of a patient from outside of the patient's body. Fluoroscopy is typically performed by introducing a catheter into a bodily passageway of the patient, for example, through one of the vessels in the leg. The catheter is a surgical instrument for withdrawing fluid from or introducing fluid or various medical devices into the body of a patient. In fluoroscopy, contrast media is introduced into the vessel that is to be observed. The catheter and the contrast media used in such procedures are well known in the art and are only described here generally for background.
After the contrast media is introduced and delivered through the blood vessel of the patient, physicians are able to see the state of the vessel. By using X-ray, physicians are able to spot possible areas of stenosis in the blood vessel. Once a narrowed portion of a blood vessel is identified, a stent is introduced to the spot of the clogged or narrowed blood vessels in order to provide a structural support. The stent generally expands within the vessel until it contacts the vessel wall. However, if the stenosis of the vessel is particularly acute, for example, greater than fifty percent, or if the substance causing the stenosis is calloused, the stent may not be able to fully expand. In those situations, a balloon is inserted to predilate the vessel. Upon implantation, the stent provides permanent structural support so that the blood vessel remains dilated. Using stents has the distinct advantage of providing both patients and physicians with a non-invasive method of treating problems of internal organs and blood vessels over traditional surgery.
Two types of the stents are generally employed: balloon-expandable stents and self-expandable stents. Although balloon-expandable stents have been used prior to introduction of the self-expandable stents, both types of the stents are currently used by physicians, depending on the area to be treated, the size of the vessel lumen, the state of the stenosis and the physician's experience and preference. For example, balloon-expandable stents are often preferable for treating critical arteries that are unlikely to experience pressure from external traumas. One of the advantages of balloon-expandable stents is that the implanted diameter of the stent can be precisely controlled. However, a disadvantage of balloon-expandable stents is that they can be catastrophically deformed when force from an external trauma is applied to the stent. As a result, the balloon-expandable stents are well suited for coronary vessels as opposed to peripheral vessels, which are frequently subject to external traumas.
With balloon-expandable stents, a balloon with a stent mounted thereon is introduced through a catheter and is inflated at the site of the narrowed vessel. The stent expands until it contacts and presses against the vessel wall. In contrast, self-expandable stents are capable of expanding without the use of a balloon. Self-expandable stents are generally made of spring metal, for example, nitinol or stainless steel. Spring metal used for making self-expandable stents typically has shape memory characteristics. Self-expandable stents are readily deformable by pressure applied thereto but return to their original shape like a spring after the pressure is removed.
Accordingly, a typical procedure for implanting a self-expandable stent is as follows. First, the stent is delivered to the vessel area to be treated through a catheter in a compressed or collapsed state. In order to keep the stent compressed during delivery, a stent delivery system having a sheath and a holder is used. In general, the sheath is an outer cover and the holder is located inside the sheath. The sheath and the holder typically have a tubular shape. The self-expandable stent is interposed between the sheath and the holder. The sheath applies pressure to the stent, thereby maintaining the stent in the collapsed state. The holder supports and carries the stent until it is released from the stent delivery system. The stent delivery system is loaded into the catheter and delivered to the vessel where the stent is to be deployed.
Second, once the stent is positioned at the area to be treated, the stent is implanted by releasing the stent from the delivery system. To release the stent, the sheath is retracted so that the stent is exposed to the vessel. A holder band is typically positioned at the proximal end of the stent. The holder band basically prevents the stent from moving rearward as the sheath is retracted. Because the sheath directly contacts and presses against the stent, the stent has a tendency to move backward as the sheath. Thus, the holder band can restrain such movement of the stent, thereby allowing the stent to be released as the sheath is retracted. Once the sheath is withdrawn, the stent is freed from pressure and starts to expand. After the stent fully expands, the stent delivery system and the catheter are removed from the vessel.
The description of balloon-expandable stents and self-expandable stents is provided for general background only. However, regardless of the type of the stent that is used, physicians have many different preferred procedures and variations for implanting stents. Further, the techniques used for implanting stents are still developing and changing. In addition, procedures usually associated with one type of the stent may also be used with different types of the stents.
During the process of deploying a stent, a conventional stent delivery systems have difficulty in properly positioning the stent. As described above, the sheath first retracts to expose the stent. During that process, the stent may act like a spring. As a result, the stent may jump out of the sheath and be starting to expand. This problem most commonly occurs where the stent is a short stent. Short stents may quickly expand before the sheath is fully retracted. Once the stent starts to expand, it is not possible to compress the stent back into the delivery system in order to reposition the stent. Also, it is difficult to adjust the position of the stent once the stent contacts the wall of the vessel. Accordingly, a stent delivery system is desirable that is able to release a stent in a controlled manner upon deployment.
Currently, most stent delivery systems on the market do not have a controlled release mechanism. Accordingly, physicians must exercise extreme caution by slowly deploying the stent so that the distal end of the stent has time to contact the vessel wall before the proximal end of the stent exits the sheath. This method is inherently unreliable and is susceptible to uncontrolled release of the stent during deployment.
In U.S. Patent Publication Nos. 2002/0120322 and 2002/0120323, a similar problem was addressed. In these patent publications, an interlock system that engages male interlock structures on the stent with female interlock structures on an inner tubular member is described. As shown in FIG. 6A of the Publications, the male interlock structures are positioned at the proximal and distal ends of the stent. The female interlock structures are formed to receive and be engaged with the male interlock structures. When the stent is in the collapsed state, the male and female interlock structures are coupled to each other.
When the sheath is retracted, the stent remains in the collapsed state until it is fully exposed. As the stent expands radially, the male interlock structures are free to radially move out of the female structures. By using this interlock system, it is possible to prevent the stent from moving longitudinally during expansion of the stent. However, this interlocking system adds extra costs in manufacturing the male and female interlocking structures. The male and female interlocking structures need to fit into each other accurately. Considering the size of the stent, the addition of male and female interlocking structures may require costly and burdensome processes. In addition, the stent may be subject to additional friction due to the interlocking structures. For example, if the interlocking structures are engaged with each other too tightly, the stent may be hindered from moving out of the stent delivery system.
U.S. Pat. No. 6,077,295 discloses a self-expandable stent delivery system. The system allows a physician to recapture a partially deployed stent. The physician can partially deploy the stent, and if the position of the stent is incorrect, the physician can manipulate control handles so that the sheath and the holder of the stent delivery system move axially in opposite directions. Specifically, the control handles can be manipulated so that the holder moves in a proximal direction, whereas the sheath moves in a distal direction. This system works on the assumption that it is possible to perform accurate manipulation of the control handles. However, it is uncertain that physicians can accurately manipulate the control handles once the stent is partially deployed. Thus, this system is still prone to errors in positioning the stent during deployment.